The purpose of candling eggs is to examine the eggs interior for signs of development or other issues.
To candle an egg you shine a bright light through the egg shell to check if the egg is fertile, to monitor the growth of an embryo inside an incubated egg and identify any problems with the egg like blood spots, cracks or non viable embryos, which allow them to be removed to prevent contamination.
You can do the candling of the eggs by using a bright flashlight in a dark room and place the egg against the light to see the internal details like the air cell and veins.
You'll see blood pumping through the heart of a tiny, developing embryo if you candle a fertile egg on Day 4.
If the embryo dies at this point, you may still see a faint network of blood vessels inside the egg's contents.
The white string in an egg is "chalazae", which are twisted, cord like strands of protein that anchor the yolk to the egg white and keeps it centered and protected.
Chalazae, which is the white string in the egg is a normal part of the egg and is also edible.
The chalazae in the egg also indicates that the egg is fresh and you don't need to remove them unless you want too.
Some people strain out the chalazae from the egg when making custard and other perfectly smooth dishes.
Chalazae in the eggs are protein cords and rope like structures attach to the egg yolk in the center of the egg to prevent it from being damaged by the egg shell.
In a fresh egg, the chalazae is more prominent and visible.
And as the egg ages, the chalazae and the egg white will break down and become less apparent.
Chalazae are very normal and safe to eat and the chalazae also add to the protein content of the egg, which makes them beneficial.
For most dishes you don't need to remove the chalazae, but for recipes which require a smooth consistency such as curds or custards you can easily remove the chalazae by straining the egg yolk or by picking them out with a fork or some kitchen tools.